Dovetail tome for implanting spinal fusion devices

ABSTRACT

A tome for cutting at least one dovetail in bone is disclosed wherein said tome comprises:  
     A) a shaft having first and second ends, said first end having attached thereto a blade shaped for cutting said dovetail in said bone and said second end having attached thereto an extension for engagement with mechanical energy transmission devices, and  
     B) a depth stop on said shaft between said blade and said extension.

[0001] This application is a continuation of application No. Ser.09/408,762, filed Sept. 30, 1999 and incorporated herein by reference,which application is a divisional of application Ser. No. 09/248,151,filed Feb. 10, 1999, which in turn is a continuation-in-part ofapplication Ser. No. 09/072,777, filed May 6, 1998. This applicationclaims the benefit of priority of those applications.

BACKGROUND OF INVENTION

[0002] 1. Field Of Invention

[0003] This invention relates generally to the treatment of injured,degenerated, or diseased tissue in the human spine, for example,intervertebral discs and vertebrae themselves. It further relates to theremoval of damaged tissue and to the stabilization of the remainingspine by fusion to one another of at least two vertebrae adjacent ornearly adjacent to the space left by the surgical removal of tissue.More particularly, this invention relates to the implantation of deviceswhich can be inserted to take the structural place of removed discs andvertebrae during healing while simultaneously sharing compressive loadto facilitate bony fusion by bone growth between adjacent vertebrae toreplace permanently the structural contribution of the removed tissue.This invention further relates to the implantation of devices which donot interfere with the natural lordosis of the spinal column. Thisinvention further relates to surgical instruments for preparing thepatient for recieving such implants.

[0004] 2. Background of the Invention

[0005] For many years a treatment, often a treatment of last resort, forserious back problems has been spinal fusion surgery. Disc surgery, forexample, typically requires removal of a portion or all of anintervertebral disc. Such removal, of course, necessitates replacementof the structural contribution of the removed disc. The most commonsites for such surgery, namely those locations where body weight mostconcentrates its load, are the lumbar discs in the L1-2, L2-3, L3-4,L4-5, and L5-S1 intervertebral spaces. In addition, other injuries andconditions, such as tumor of the spine, may require removal not only ofthe disc but of all or part of one or more vertebrae, creating an evengreater need to replace the structural contribution of the removedtissue. Also, a number of degenerative diseases and other conditionssuch as scoliosis require correction of the relative orientation ofvertebrae by surgery and fusion.

[0006] In current day practice, a surgeon will use one or moreprocedures currently known in the art to fuse remaining adjacent spinalvertebrae together in order to replace the structural contribution ofthe affected segment of the disc-vertebrae system. In general for spinalfusions a significant portion of the intervertebral disk is removed, andif necessary portions of vertebrae, and a stabilizing element,frequently including bone graft material, is packed in theintervertebral space. In parallel with the bone graft material,typically additional external stabilizing instrumentation and devicesare applied, in one method a series of pedicle screws and conformablemetal rods. The purpose of these devices, among other things, is toprevent shifting and impingement of the vertebrae on the spinal nervecolumn. These bone graft implants and pedicle screws and rods, however,often do not provide enough stability to restrict relative motionbetween the two vertebrae while the bone grows together to fuse theadjacent vertebrae.

[0007] Results from conventional methods of attempting spinal fusionhave been distinctly mixed. For example, the posterior surgical approachto the spine has often been used in the past for conditions such asscoliosis, using Harrington rods and hooks to align and stabilize thespinal column. In recent years many surgeons have adopted anteriorfusion because of the drawbacks of the posterior approach, the primaryproblem being that in the posterior approach the spine surgeon mustnavigate past the spinal column and its nerve structure. However,results of anterior surgery are variable and uncertain becauseconstraining the vertebrae from this side does not address the loads puton the spine by hyperextension, such as from rocking the body in abackwards direction.

[0008] Pedicle screws and rods, always implanted posteriorly, tend toloosen either in the bone or at the screw-rod interface if fusion is notobtained. Fusion rates for posterolateral instrumented fusions rangefrom 50% to 90%. It must be kept in mind that plain x-rays are only65-70% accurate in determining fusion status and most studies use thisinadequate method to determine fusion status, suggesting that thenon-union rate may be greater than reported. It is also known thatposterior pedicle screw systems do not prevent all motion anteriorly,leading to the risk of fatigue failure of the metal and screw breakage.This continued motion may also lead to persistent pain, despite solidposterior bony fusion, if the disc was the original pain generator.These well documented failures of pedicle screws have given rise toextensive litigation in the United States.

[0009] It is well established from the study of bone growth that a bonewhich carries load, especially compressive load, tends to grow andbecome stronger. Existing stabilizing implants, in particular IBF's, donot share any of the compressive load with the new bone growth, in factpossibly shielding new bone growth from load.

[0010] The biggest limitation in any method of fusion at the presenttime is the nature of available devices for bridging the space left byexcision of diseased or damaged tissue. In particular, interbody fusion(IBF) devices currently on the market in the United States do notprovide stability in all planes of motion. There is very little evidenceto support the biomechanical stability of these devices. They aregenerally stable in compression (forward flexion) unless the bone isosteoporotic, which condition could lead to subsidence of the deviceinto the adjacent vertebral body with loss of disc space height. Theymay be much less stable in torsion and certainly less so in extensionwhere there is no constraint to motion except by the diseased annulusfibrosus which is kept intact to provide just such constraint. It isdoubtful that a degenerative annulus could provide any long term“stiffness” and would most likely exhibit the creep typically expectedin such fibro-collagenous structures.

[0011] Accordingly, there is widespread recognition among spine surgeonsof the need for a flexible radiolucent implant device which wouldreplace removed degenerated tissue and be firmly affixed mechanically toopposing vertebrae. Such a device would dramatically increase theprobability of successful fusion because it a) would eliminate orsignificantly reduce relative movement of the adjacent vertebrae and theintervertebral fixation device in extension and torsion, b) wouldthereby reduce or eliminate the need for supplemental external fixation,c) by compressive load sharing would stimulate rapid growth of the boneelements packed within the intervertebral device by causingosteoinduction within the bone chips, thereby accelerating fusion, d)would allow confirmation that fusion had taken place using standard CTor possibly plain x-rays, and e) would have the potential to bebioabsorbable, potentially being fabricated from such materials as aD-LPLA polylactide or a remodelable type-two collagen so as to leave inthe long term no foreign matter in the intervertebral space. Inaddition, a flexible implant device can be fabricated in whole or inpart from human bone autograft or from bone allograft material which issterilized and processed, automatically approximately matching theelastic properties of the patient's bone. The success rate of fusionusing such an approach is anticipated to exceed the success rate of theIBF devices or the external fusion devices alone and at least equal thecombined success rate of the current combination IBF and posteriorinstrumented technique.

[0012] Lordosis, which is a pronounced forward curvature of the lumbarspine, is a factor which needs to be taken into account in designinglumbar implants. It is known in the art that preserving the naturalcurvature of the lumbar spine requires designing into a new device suchas the current invention a modest taper approximately equivalent to theeffective angularity of the removed tissue. The restoration of normalanatomy is a basic principle of all orthopedic reconstructive surgery.

[0013] Therefore there is a perceived need for a device whichsimultaneously and reliably attaches mechanically to the bony spinalsegments on either side of the removed tissue so as to prevent relativemotion in extension (tension) of the spinal segments during healing,provides spaces in which bone growth material can be placed to create orenhance fusion, and enables the new bony growth, and, in a graduallyincreasing manner if possible, shares the spinal compressive load withthe bone growth material and the new growth so as to enhance bone growthand calcification. The needed device will in some instances require amodest taper to preserve natural lumbar spinal lordosis. It will also beextremely useful if a new device minimizes interference with orobscuring of x-ray and CT imaging of the fusing process. Based on suchneed, there is also obviously a need for the surgical tools necessary toprepare the intervertebral space and insert the type of implant which iscovered in the parent application.

[0014] Thus it is an object of the current invention to provide astabilizing device for insertion in spaces created between vertebraeduring spinal surgery. It is a further object to create an implantabledevice for stabilizing the spine by preventing or severely limitingrelative motion between the involved vertebrae in tension (extension)and torsion loading during healing. It is a further object to provide adevice which promotes growth of bone between vertebrae adjacent to thespace left by the excised material by progressive sharing of thecompressive load to the bone graft inserted within the device. It is yeta further object to provide mechanical stability between adjacentvertebrae while bone grows through a lumen in the implant and at thesame time not diminish the natural lordosis of the lumbar spine. It is afurther object of the invention to provide a device which avoids orminimizes interference with various imaging technologies. It is anotherobject of this invention to be capable of being fabricated from humanbone allograft material. It is yet a further object of this invention toprovide the surgical tools necessary for implantation of such devices.In particular, it is an object of the invention of this application toprovide the surgical tools necessary to prepare the intervertebral spaceand the mating grooves for an interlocking implant.

SUMMARY OF THE INVENTION

[0015] The invention disclosed here is a cutting tool for preparingdovetail grooves in adjacent vertebrae to receive the novel implant ofthe parent application. The design of the new implant for spinal surgeryincludes the possibility of fabricating the device out of material whichis elastic, especially in response to compressive loads, preferably witha compressive elasticity closely matched to that of human bone,preferably the patient's bone. In particular, the design includes thecapability to fabricate the device from human bone allograft material.The design is also such that the implant mechanically fastens or locksto adjacent vertebrae and stabilizes the involved vertebrae in tensionand in torsion while transmitting a portion of the vertical compressiveload to new bone growth associated with the device. This feature of theinvention will cause osteoinduction within the bone chips loaded intothe implant and will share a sufficient portion of the load withexisting bone and with the new bone growth to promote further bonegrowth and not interfere with bone fusion growth. This invention can betapered to preserve natural lordosis. This invention also minimizesinterference with x-ray imaging by virtue of being fabricated in wholeor in part from radiolucent materials.

[0016] The implant joins two vertebrae by means of a mechanical fixationdevice which is hollow to allow bone growth matter to be added to one ormore spaces communicating with the top and bottom surfaces for thepurpose of promoting fusion. The attachment portion of the mechanicalfixation device is, in a first embodiment, a tongue and groovemechanical fastening arrangement. Other mechanical fasteners commonlyused in the woodworking art, such as tack and staple devices, can alsobe used. The mechanical properties of the device are closely matched tothe bone's modulus of elasticity so as to promote osteoinduction andrapid bone growth. The devices are generally transparent to existingradiologic imaging techniques so as to allow follow up confirmation offusion of the adjacent vertebrae. The implant can also be fabricatedfrom bioabsorbable materials so as to leave no long term foreign matterin the body. Human bone allograft material can also be used as thematerial from which the implant device is fabricated.

[0017] In its most general form, the implant mechanically attaches tothe ends of and promotes bony fusion between at least two vertebraeadjacent to a space left by surgically removed spinal tissue, comprisinga load-sharing body comprising a structure having a combination ofstructural elements fabricated from at least one material having agreater than zero elastic compliance, the combination comprising atleast a top surface and a bottom surface; said combination of structuralelements establishing for the structure as a whole a composite greaterthan zero elastic compliance at least in compression in directionsgenerally axial to said top surface and said bottom surface; saidcombination of structural elements further comprising at least onecavity communicating with both said top surface and said bottom surfacein a configuration suitable as a receptacle for bone implant and growthmaterial; and on each of said top surface and said bottom surface atleast one fastener capable of mechanically anchoring the body to saidadjacent vertebrae and thereby transmitting tensile and torsional loadsto and from said adjacent vertebrae.

[0018] In another embodiment, the implant mechanically attaches to theends of and promotes bony fusion between at least two vertebrae adjacentto a space left by surgically removed spinal tissue, comprising astructure formed from a single piece of bone allograft material andhaving a top and a bottom, said structure having an internal cavitycommunicating with said top and said bottom for receiving autograft boneimplant material and bone growth factors, said unitary structure havingat least one dovetail tongue protrusion on each of said top and saidbottom for mechanically interlocking with said adjacent vertebrae byforming a mechanical tongue-and-groove joint.

[0019] In yet a third embodiment, the implant mechanically attaches tothe ends of and promotes bony fusion between at least two vertebraeadjacent to a space left by surgically removed spinal tissue, comprisinga composite structure fabricated with at least two separate portions ofbone allograft material with different structural properties and havinga top and a bottom, said structure having an internal cavitycommunicating with said top and said bottom for receiving autograft boneimplant material and bone growth factors, said unitary structure havingat least one dovetail tongue on each of said top and said bottom formechanically interlocking with said adjacent vertebrae.

[0020] Perhaps the most important aspect of the implant procedure is thepreparation of the space to receive the implant and the grooves for thedovetail fasteners. A cutting jig is used which distracts the vertebraeand stabilizes them during preparation and acts as a guide for precisecutting. The invention of this application comprises a special tomespecifically designed to precisely cut a dovetail shaped groove in theadjacent vertebrae and to prepare the end plate surfaces. The tome orcombination of tomes has an offset which provides for the implant to besized to slide through the jig but fit very tightly in the space cutinto the vertebrae so as to prevent backout of the implant. Once thecutting jig is in place an x-ray is taken to show that the end of thedistraction tangs are clearing the spinal canal. The tomes have depthstops which prevent cutting beyond the distraction tangs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1A is a frontal view of an implant placed between lumbarvertebrae.

[0022]FIG. 1B is a side view of the same implant.

[0023]FIG. 2 is a plan view of the same implant.

[0024]FIG. 3A is a plan view of an implant showing cavitiescommunicating with top and bottom surfaces into which bone growthmaterial is placed.

[0025]FIG. 3B is a frontal view of the same implant showing cavities.

[0026]FIG. 3C is a side view of the same implant showing cavities.

[0027]FIG. 4A shows a composite implant with inset titanium endplates inplan view.

[0028]FIG. 4B is a frontal view of a composite implant with insettitanium endplates.

[0029]FIG. 4C is a side view of a composite implant with inset titaniumendplates.

[0030]FIG. 5 is an isometric representation of the second embodimentusing a horseshoe shaped tongue and groove dovetail fastener and showingthe retaining barb.

[0031]FIG. 6 shows the implant of FIG. 5 inserted between adjacentvertebrae.

[0032]FIG. 7 is an isometric view of a modular implant.

[0033]FIG. 8 is an isometric view of the same modular implant withpartial depiction of adjacent vertebrae.

[0034]FIG. 9 shows an implant with a retaining barb.

[0035]FIGS. 10A and 10B depict the handle of the emplacement instrumentsfor preparation of the implant site.

[0036]FIGS. 11A and 11B show further details of a cutting tool orinstrument for preparation of the implant site.

[0037]FIGS. 12A and 12B show the operation of the interlock mechanismfor the cutting instrument for preparation of the implant site.

[0038]FIGS. 13A, 13B, and 13C show the cutting instrument forpreparation of the implant site with dovetail tome deployed.

[0039]FIGS. 14A and 14B display details of the dovetail tome.

[0040]FIG. 15 is an isometric view of the driver.

[0041]FIGS. 16A and 16B show detail of the placement implement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] In the currently preferred embodiments of the implants, torsionaland tensional stability of the spine are provided by fastenerscomprising dovetail joints which engage grooves cut during surgery inthe vertebrae adjacent to the removed tissue such that the implant andwhich has large surface contact areas. The dovetails transfer extensionand torsional loads between the two vertebrae and the flat contactsurface transmits the compressive loads. The device further comprisesone or more holes through and/or cavities inside the implant such thatthe spaces created can be filled with bone graft material which willgrow into and attach to the healthy vertebral bone. Optionally in allembodiments tapers to accommodate natural lumbar lordosis can beincorporated as necessary.

[0043] In this discussion, we use for convenience a definition of“elastic compliance” as the elastic displacement per unit of appliedforce, in other words the reciprocal of stiffness. The composite elasticcompliance of the device is selected at a value which promotes sharingof compressive load with bone graft and growth material and new bonygrowth. As discussed at greater length below, in one embodiment, humanbone allograft material is used to fabricate the implant. The new fusionbone will gradually share an increasing portion of the compressive loadsexperienced by the spine because the implant is made of a material, suchas a polymer, which has a compressive modulus which works in conjunctionwith the implant design to closely match the modulus of elasticity ofbone during deformation under load. The polymer, or in one embodimenthuman bone allograft material, has the added advantage of beingtransparent in x-ray imaging permitting, easy visualization of thefusion process at the vertebral interface. In a variant of oneembodiment, metal retaining clips may be located in the implant surface,both above and below the dovetails, to engage the cortical bone andprevent the implant from migrating out of the intervertebral space. Theretainers will generally be metal in order to benchmark x-ray imagingfor locking engagement assessment. In yet another variation, lockingbarbs will be included on the implant top and bottom surfaces to assistin securing the implant to adjacent bony surfaces to minimize pullout.

[0044] In a second embodiment of the implant, a plurality of dovetailprotrusions, or a compound dovetail protrusion in the approximate layoutof a horseshoe may be located on the outboard portions of the implant,thereby utilizing the strength and rigidity of the vertebrae to supportthe spinal column load. In this case the device would contain a hollowcentral core which would be filled with bone chip and biological mediumto accelerate the fusion in the intervertebral space.

[0045] In the first preferred embodiment, as shown in FIGS. 1A and 1B(elevation views), vertebrae L4 and L5 (or vertebrae L5 and S1) aremechanically attached by the implant 3. The device 3 is heldmechanically to the adjacent vertebrae 1 and 2 by tongue and groove, ordovetail, arrangements 4. As shown in FIG. 2 (plan view), the implant 3is sited so as to provide mechanical support to the spine both incompression and in tension, but not so as to intrude into the space 6occupied by the spinal nerve bundle. In this preferred embodiment, asshown in FIG.2, the implant 3 will include penetrations or holes 7 thepurpose of which is to contain bone growth material to facilitate bonyfusion of the adjacent vertebrae. The implant itself may comprise avariety of presently acceptable biocompatible materials such asPolyphenolsulfone, Polyetheretherketone (PEEK), Polysulfone, Acetal(Delrin), UHMW Polyethylene, and composites of these materials involvinghigh strength carbon fibers or REM glass filaments to add tensile andshear strength. As discussed more extensively below, the implant mayalso be fabricated from human bone allograft material, autograftmaterial, or bone substitute material, such as coral or calciumphosphate. The body of the implant may optionally have a modest taper toaccommodate the natural lordosis of the lumbar spine.

[0046] One possible problem with an implant with dovetail fastenersfabricated from a material such as polysulfone is that torque on oneadjacent vertebra relative to the other may place large tension stresseson the angular portions of the dovetail, thereby causing breaking andcrazing of the polysulfone. Thus a variation on this embodimentcomprises a composite implant fabricated from plastic material such aspolysulfone for the body and titanium for endplates bearing the dovetailprotrusions.

[0047]FIGS. 3A, 3B, and 3C show one possible arrangement of such acomposite structure, with a titanium endplate 8 set into the plastic(and radiolucent) body 9. FIGS. 4A through 4C show a variation on thisarrangement with the endplate extending to the shoulders of the plasticbody of the implant 11. Both FIGS. 3 and 4 show a variation of thisstructure, with the titanium endplate 12 set into the plastic body ofthe implant 9 and 11 in a configuration designed to provide throughspaces or cavities 14 in which to place bone growth material. In theselatter configurations, the polysulfone body is insert molded into thetitanium endplates. The titanium dovetail fasteners possess the tensilestrength necessary to avoid fracture or crazing, but the body is still“see through” with respect to X-ray and other methods of visualizinghealing progress. In addition, holes in the titanium endplates which arealigned with the bone growth material cavities provides “see through”capability in the vertical direction for assessing new bone growth.

[0048] A second major preferred embodiment, shown in isometric view inFIG. 5, is inserted between two vertebrae, e.g., L4 and L5 or L5 and S1and mechanically attached by two or more dovetail joints, or by acompound horseshoe shaped dovetail, located on each of the top andbottom surfaces of the implant to the adjacent remaining vertebrae by acomposite tongue and groove mechanism similar to but larger than thatused to secure the implant of the previous embodiment. In thisconfiguration, the implant comprises either a horseshoe shaped dovetailtongue 33 which in effect creates two dovetail joints per surface towardthe outboard ends of the implant top and bottom surfaces or simply twooutboard dovetail tongues without the horseshoe top closure. Thehorseshoe top closure may be substantially curved or it may besubstantially straight, with relatively square corners where thedovetail tongue angles back into the body of the vertebra. In avariation on this embodiment, inside the horseshoe shaped dovetailtongue protrusion 33 the body of the implant is hollow, that is, itcontains an opening or cavity 34 communicating with both the top surfaceand the bottom surface into which bone growth material is placed.

[0049] In this preferred embodiment, as further shown in the isometricview of FIG. 6, the implant 35 with a relatively squared off horseshoetop closure will have a surface approximately flush with the exteriorsurface of the adjacent vertebrae and will appear to create one verywide dovetail 37. This embodiment of the implant will also includepenetrations or holes in addition to or as an alternative to that shownin FIG. 5, 34, the purpose of which is also to contain bone growthmaterial to facilitate bony fusion of the adjacent vertebrae. As in theprior configuration, the implant 35 is sited so as to provide mechanicalsupport both in compression and in tension to the spinal column, but notso as to intrude into the space 6 occupied by the spinal nerve bundle.The implant in some cases is further inserted inside remaining segmentsof intervertebral disc tissue 38. As shown in both FIGS. 5 and 6, anoptional feature of these embodiments is for the faces of the implant tohave locking barbs 36 to retain the implant in place between theremaining vertebrae once it is inserted.

[0050] This implant, as in the prior embodiment, may itself comprise avariety of presently acceptable implant materials such as PEEK(Polyetheretherketone), acetal (DELRIN), polysulfone, Ultra HighMolecular Weight Polyethylene (UHMW Poly), and composites involving highstrength carbon fibers or glass filaments to add tensile and shearstrength. Again, as discussed at greater length below, human boneallograft material may be used to fabricate this device. This embodimentmay also be fabricated with a modest taper to accommodate naturallordosis.

[0051] A third preferred embodiment of the lumbar implant, shown inisometric view in FIG. 7, comprises three elements, two modular dovetailhalves, 41 and 42, which are inserted between vertebrae L4 and L5 or L5and S1 and mechanically attached by two dovetail protrusions (similar tothose fabricated for the second embodiment) located on the top andbottom of the implant to the adjacent vertebrae by a tongue and groovemechanism similar to but larger than that used to secure previousembodiments of the implant. The two modular dovetail halves are heldtogether by a retainer 43. As in the prior configuration, as shown inthe isometric view of FIG.8, the implant 35 is sited so as to providemechanical support both in compression and in tension to the spinalcolumn, but not so as to intrude into the space 8 occupied by the spinalnerve bundle.

[0052] In this preferred embodiment, as shown in FIG.8, the implant 35will include a cavity 39 the purpose of which is to contain bone growthmaterial to facilitate bony fusion of the adjacent vertebrae. The openspace 39 is packed with bone growth material and then capped with aretainer, 43, designed to snap in place to add stability to the implantand to retain the bone growth factor to prevent it from migrating. Thisimplant, as in the prior embodiment, may itself comprise a variety ofpresently acceptable implant materials such as PEEK (Polyesther EstherKetone), Acetyl (delrin), polysulphone, Ultra High Molecular Weightpolyethylene (UHMW Poly), and composites involving high strength carbonfibers or glass filaments to add tensile and shear strength. Again themodular dovetail halves may be tapered to accommodate lordosis.

[0053] Any of the foregoing embodiments can additionally have a featureshown in FIGS. 5, 6, and 9, namely a retractable barb 36. This barbcomprises a spring wire which when deployed engages the adjacentvertebrae to prevent the implant from dislodging. A retraction tool maybe inserted into the hole 39 to cause the sigma-shaped barb to retractits probe-like end so that the implant disengages from the adjacentvertebra.

[0054] As previously noted, any of the foregoing embodiments of theCor-Lok™ interlocking implant can be fabricated from cadaver bone whichis processed to form bone allograft material. Tissue grafting of livingtissue from the same patient, including bone grafting, is well known.Tissue such as bone is removed from one part of a body (the donor site)and inserted into tissue in another (the host site) part of the same (oranother) body. With respect to living bone tissue, it has been desirablein the past to be able to remove a piece of living tissue graft materialwhich is the exact size and shape needed for the host site where it willbe implanted, but it has proved very difficult to achieve this goal.

[0055] On the other hand, processing of bone material which does notcontain living tissue is becoming more and more important. Non-livingbone grafting techniques have been attempted both for autografts and forallografts. For example, Nashef U.S. Pat. No. 4,678,470 discloses amethod of creating bone graft material by machining a block of bone to aparticular shape or by pulverizing and milling it. The graft material isthen tanned with glutaraldehyde to sterilize it. This process canproduce bone plugs of a desired shape.

[0056] In the Nashef process, the process of pulverizing or milling thebone material destroys the structure of the bone tissue. The step oftanning it with glutaraldehyde then renders the graft materialcompletely sterile.

[0057] In the prior art, inventors have believed that it is desirable tomaintain graft tissue in a living state during the grafting process.There is no doubt that the use of living tissue in a graft will promotebone healing, but much surgical experience has shown that healing can beachieved with allografts of non-living bone material which has beenprocessed. In fact, spine surgeons express a distinct preference forsuch materials, and at least one supplier, the MusculoskeletalTransplant Foundation (MTF), has introduced femoral ring allografts forspine surgeries.

[0058] It is now possible to obtain allograft bone which has beenprocessed to remove all living material which could present a tissuerejection problem or an infection problem. Such processed materialretains much of the structural quality of the original living bone,rendering it osteoinductive. Moreover, it can be shaped according toknown and new methods to attain enhanced structural behavior.

[0059] Research shows that such allografts are very favorable for spinalsurgery. According to Brantigan, J. W., Cunningham, B. W., Warden, K.,McAfee, P. C., and Steffee, A. D., “Compression Strength Of Donor BoneFor Posterior Lumbar Interbody Fusion,” Spine, Vol. 18, No. 9, pp.12113-21 (July 1993):

[0060] Many authors have viewed donor bone as the equivalent ofautologous bone. Nasca et al . . . compared spinal fusions in 62patients with autologous bone and 90 patients with cryopreserved boneand found successful arthrodesis in 87% of autologous and 86.6% ofallograft patients.

[0061] (Citations omitted.) Moreover, as previously noted, sources ofsafely processed allograft material have recently become available.

[0062] In the present invention, allograft bone is reshaped into one ofthe Cor-Lok™ configurations for use as a spine implant. Various methods,including that of Bonutti, U.S. Pat. Nos. 5,662,710 and 5,545,222, canbe used to shape the allograft material into the desired shape.

[0063] In the first sub-embodiment of this aspect of the currentinvention, bone material which yields to compressive loads at theexterior surfaces without significant degradation of the interiorstructural properties, such as cancellous or trabecular bone, is shaped.It is not unusual that reshaping of graft tissue is necessary to obtainthe best possible graft. In particular, bone tissue may be stronger andbetter able to bear force when it is denser and more compact.

[0064] Compression of allograft bone is desirable from generalconsiderations. Generally, bone samples are stronger when they are moredense. Compressing allograft bone increases its density and thusgenerally strengthens the allograft. The allograft bone also staystogether better. In addition, recent studies have indicated that theshell of vertebral bone is very much like condensed trabecular bone.Mosekilde, L., “Vertebral structure and strength in vivo and in vitro,”Calc. Tissue Int. 1993;53(Suppl): 121-6; Silva, M. J., Wang, C.,Keaveny, T. M., and Hayes, W. C., “Direct and computed tomographythickness measurements of the human lumbar vertebral shell andendplate,” Bone 1994;15:409-14; Vesterby, A., Mosekilde, L., Gunderson,H. J. G., et al., “Biologically meaningful determinants of the in vitrostrength of lumbar vertebrae,” Bone 1991;12:219-24. Compressing boneallograft material prior to implantation thus generally produces astronger graft.

[0065] Compression also allows conversion of larger irregular shapesinto the desirable smaller shape, thereby permitting more disparatesources of allograft bone to be used. By compressing bone to a givenshape it is possible to configure the allograft to match a preformeddonee site prepared by using a shaped cutter to cut a precisely matchingcut space. In particular, this method of formation facilitates theformation of dovetail tongue protrusions on the upper and lower surfacesof the implant for the formation of a tongue-and-groove mechanical jointwith adjacent vertebrae.

[0066] In the current invention, a blank is cut from cancellous ortrabecular allograft bone and placed in a forming apparatus. The formingapparatus compresses the sample into the desired shape. In particular,this process forms the dovetail tongue protrusions on the implant upperand lower surfaces for the tongue-and-groove joint. The cancellous ortrabecular material yields at the external surface under the pressure toform a compacted layer around the outside of the allograft form. Thiscompacted layer is not destroyed material but rather forms substantiallya structure with properties of the vertebral shell or of a monococquedesign, including additional structural properties such as enhancedtensile strength. This enhanced tensile strength enables the allograftmaterial to perform the same function in resisting torsion and extensionof the spine as does the synthetic materials previously discussed. Suchprocesses in general are able to maintain the homologous property of theallograft material.

[0067] In the second embodiment of this aspect of the implant, differenttypes of allograft bone are formed into a composite structure to providethe necessary structural properties. Both cortical or shell and densecancellous or trabecular bone may be compacted into a unified structure.Fibrin “glue” is highly suitable for use as an adhesive in suchstructures. Fibrin is a blood component important in blood clotting. Itcan be separated or centrifuged from blood and has the nature of anadhesive gel. Fibrin can be used as an adhesive, either in a naturalstate or after being compressed, to hold together material such asseparate tissue pieces pressed together in a tissue press. Inparticular, cortical bone from the same source can be used as a shell toprovide needed additional structural properties, such as tensilestrength to a composite shape. Cortical bone can also be provided in ashell, much like the known femoral ring implants, to provide the neededstructural properties. Moreover, a shell is not the only structuralelement which can be added in this way. Buttresses, gussets,cross-braces, and other structural elements can be included in the sameway. Using such materials, the homologous property of the bone allograftmaterial may be maintained.

[0068] In another sub-embodiment of the implant, a relatively thinexternal shell of a synthetic material can be provided for enclosingcompressed allograft material and providing any needed additionalstructural properties. After the graft is compressed, the shell isplaced around the graft. The shell may be made of a material whichexpands after it is placed in the spine, thereby supplementing theinterlocking properties of the Cor-Lok™ mechanical design by improvingthe fill between the allograft and the donee site. There are a number ofsuitable materials which expand when they come in contact with water orother fluids. One is PEEK (polyether-etherketone)(water absorption about1.5%). A desiccated biodegradable material or suitable desiccatedallograft material may also be used.

[0069] The expansion can take place in one of two ways. First, theretainer can itself be compressed, as with the tissue, then expand whenplaced in the body. Second, the retainer can be made of a material whichexpands when it comes in contact with water or other bodily fluids.

[0070] It should be noted that the entire allograft implant can itselfbe compressed so that it expands when contacted by water. The expandableshell material can first be compressed with the allograft material,which then expands when placed in the body.

[0071] It should further be understood that the graft can be multipletissue fragments rather than a composite material. The compressingprocess can be used to compress multiple bone fragments into one largerpiece. It should also be understood that the compression process can beused to add additional materials to an allograft composite. For example,to bone tissue there can be added tri-calcium phosphate, an antibiotic,hydroxyapatite, autografts, or polymeric materials.

[0072]FIGS. 10A through 16B depict the surgical tools used to installthe implant. This apparatus comprises a set of unique tools which willaccurately cut a dovetail joint in bone for the purpose of inserting animplant which locks adjacent vertebrae together.

[0073] The guide 44, shown in FIGS. 10A and 10B, is a tubular tool withtangs 45 extending from one end. The tangs, tapered 46 to conform tonatural lordosis, are inserted between the vertebrae 47 and distractthem to a preferred dimension 48, as shown in FIG. 10B. The driver 68,shown in FIG. 15, can be used with a rod extension guide adapter 70,also shown in FIG. 15, to drive the guide 44 into place. This stepestablishes a fixed reference relative to the two vertebrae 47 andsecures the vertebrae from moving. The length 49 of the tangs 45 isconsistent with the other tools in the set and establishes the extent 49to which any tool can penetrate. A lateral x-ray is used to assure thatthe extent of penetration 49 is safely away from the spinal canal 50.All of the other tools have positive stops which contact the guide depthstop 51 to control the depth of cut.

[0074] The end cut tool 52, shown in FIGS. 11A and 11B, is inserted intothe guide 44 to make an end-cut 25, shown in FIG. 11B, for the dovetail.Once completely inserted to the depth stop 53, a single piece interlock54, shown in FIGS. 12A and 12B, which prevented rotation of the blade 55during insertion, is disengaged from the shaft 56 and then preventswithdrawal of the end cut tool 52 from the guide 44. As shown in FIGS.12A and 12B, the interlock 54 is held by spring 57 such that it engagesthe slot 58 in the shaft 56, preventing rotation as shown in FIG. 12A.As the end cut tool 52 is inserted into the guide 44 it pushes theinterlock 54, rotating it out of the slot 58 in the shaft 56 as shown inFIG. 12B. As the interlock rotates, it engages the guide 44 as shown inFIG. 12B. When the shaft 56 is rotated as shown in FIG. 12B theinterlock 54 cannot return to its original position as shown in FIG.12A, thus securing the end cut tool 52 in the guide 44. The rotationinterlock protects the surgeon from the end cut blade 55 and thewithdrawal interlock holds the end cut tool 52 in the guide 44 while theblade 55 is exposed. The surgeon rotates the handle 59 one turn, causingthe end cut blade 55 to make end-cuts 25 as shown in FIG. 11B, in bothvertebrae 47 simultaneously, and returns it to the “zero” position atwhich the end cut tool 52 can be removed from the guide 44.

[0075] The dovetail tome 60, shown in FIG. 13A, is inserted into theguide 44 to the point where the blade 61 rests against the vertebrae 47.As shown in FIG. 15, the driver 68 is placed on the dovetail tome rodextension 62 and drives the dovetail tome 60, cutting the vertebrae 47,until the depth stop 63 of the dovetail tome contacts the stop 51 on theguide 44, stopping the blade 61 at the end-cut 25, as shown in FIG. 13C.The dovetail tome blade 61, as shown in FIG. 14A, has endplate breakers64 which split the endplates 65 of the vertebrae (see FIG. 13C) in two66 as shown in FIG. 14B, preventing them from jamming in the blade andpreparing them for later use. The dovetail tome 60 is removed and thebone 67 and the split vertebral end plate 66 contained in the blade 61is harvested for later use in the implant 33.

[0076] As shown in FIG. 15, the driver 68 is a pneumatic tool like aminiature jackhammer. The driver 68 is powered by compressed gassupplied through the input tube 69. The driver 68 receives the rodextension from the guide adapter 70 or the rod extension of dovetailtome 62 into a guide port 71. A piston 72, within the driver 68,repeatedly impacts the guide adapter 70 or the dovetail tome rodextension 62, driving the tool into place. The driver 68 is activated bythe finger-actuated valve 73. Control of the force and rate of theimpacts is attained by modulating the valve 73. The driver will deliverseveral thousand small impacts in place of a few massive blows from ahammer.

[0077] The implant 33 of FIG. 5 is prepared for insertion by filling theinterior portion 34 with harvested bone 67 and the split end plates 66from the dovetail tome cuts and additional bone and graft stock. Theimplant 33 is then slid down the guide 44 (FIG. 10) and driven intoplace by the insertion tool 74, shown in FIGS. 16A and 16B. Theinsertion tool 74 has a positive stop 75 which contacts the depth stop51 of the guide 44 and assures correct placement of the implant 33,locking the vertebrae 47.

[0078] The above implant devices contain attachment means which are wellknown in the woodworking industry, but are not used in Orthopedic SpineSurgery. However, one skilled in the art of intervertebral implantswould readily be able to adapt other fastening devices known in thewoodworking art to spinal implant devices. It should be readily apparentto anyone skilled in the art that there are several available means toattach bone surfaces to the adjacent implant surfaces, such as causingbone anchors to protrude from the implant surface and impinge and attachthe adjacent vertebrae to the implant. Metal staple-like clips can bedriven between adjacent vertebrae to attach the edges of the vertebrae.Tack and staple configurations can substitute for the dovetail tongueand groove fasteners. Bone anchors can also be used to attach naturaltissue to adjacent vertebrae, creating an artificial ligament whichcould scar down, thus retaining an artificial implant within the discspace while osteoinduction takes place and the vertebrae fuse.

We claim:
 31. A tome for precisely cutting in at least one bone segmentat least one groove having the shape of an intimately mating dovetail,said tome comprising a shaft having first and second ends, said firstend having attached thereto a blade shaped for cutting said dovetailgroove in said bone and said second end having attached thereto anextension for engagement with mechanical energy transmission devices,said blade having a shape comprising a plurality of substantially planarsegments.
 32. The tome of claim 31 wherein the blade having a shapecomprising a plurality of substantially planar segments furthercomprises short radius curves connecting said plurality of substantiallyplanar segments into a single piece.
 34. The tome of claim 33 whereinthe blade configured to cut a first dovetail groove in a first vertebraand a second dovetail groove in a second vertebra simultaneously isreplaced by an assembly of blades configured to cut a first dovetailgroove in a first vertebra and a second dovetail groove in a secondvertebra simultaneously.